Systems and methods for securing a digital communications link

ABSTRACT

A digital data signal, such as a digital video signal, is intentionally pre-distorted before being sent over a network. In one embodiment, this pre-distortion may be performed in accordance with a pre-distortion pattern or algorithm which is shared with only intended receivers. The pre-distortion pattern may be used to vary the pre-distortion on a periodic basis, as frequently as on a symbol-by-symbol basis. The pre-distortion function may include distorting the phase and/or the amplitude of the digital signal&#39;s modulation.

FIELD OF THE INVENTION

The invention relates in general to securing communications, and inparticular, to securing communications link based on intentional signalpre-distortion, pre-emphasis or even varying modulation schemes.

BACKGROUND OF THE INVENTION

Communications, whether wireline or wireless, often involve the transferof sensitive information. In order to avoid exposing or otherwiseallowing third-party access to such information, sensitivecommunications are typically encrypted using various known cryptographicalgorithms, such as the Advanced Encryption Standard (AES), DataEncryption Standard (DES), etc. However, use of such cryptographicalgorithms involve both encrypting and decrypting communications, whichtends to contribute a relatively large amount of processing overhead tothe overall communication process. This can be particularly burdensomein the context of high-definition video content which alone requires arelatively high amount of processing power to perform both the encodingand decoding functions.

In addition, particularly in the wireless context, the quality of thecommunication channel can quickly degrade. Such signal distortion mustbe compensated for on the receiver-side before the signal can beproperly demodulated. Thus, in order to properly receive an encryptedvideo signal, both a signal distortion correction operation and adecryption operation must be performed before the signal itself can evenbe demodulated in accordance with whatever video modulation scheme isbeing used (e.g., Binary Phase-shift Keying (BPSK), Quadraturephase-shift keying (QPSK), Quadrature amplitude modulation (QAM),Orthogonal Frequency-Division Multiplexing (OFDM), etc.). The end resultis that relatively complex and expensive hardware is required on thereceiver-side in order to accurately process the incoming encryptedvideo stream. Thus, there is a need for a system and method for securinga communication link that do not rely on traditional encryption schemes.

SUMMARY OF THE INVENTION

Disclosed and claimed herein are methods, systems and devices forproviding secure digital communications. In one embodiment, a methodincludes receiving a pre-distortion pattern, receiving a pre-distorteddata signal over a network connection, and undistorting at least one ofa phase and an amplitude of the pre-distorted data signal in accordancewith said pre-distortion pattern.

Other aspects, features, and techniques of the invention will beapparent to one skilled in the relevant art in view of the followingdetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1A depicts a typical BPSK constellation diagram without distortion;

FIG. 1B depicts a typical BPSK constellation diagram with distortion;

FIGS. 2A-2B depict a BPSK constellation diagram for one embodiment ofthe invention;

FIG. 3A depicts a typical QPSK constellation diagram of the prior art;

FIG. 3B depicts a QPSK constellation diagram for one embodiment of theinvention;

FIGS. 4A-4B depict simplified system-level diagrams for one or moreembodiments of the invention;

FIG. 5 is one embodiment of a process for implementing one or moreaspect of the invention; and

FIG. 6 is another embodiment of a process for implementing one or moreaspect of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS Disclosure Overview

One aspect of the present disclosure relates to pre-distorting themodulation of a digital data signal, such as a digital video signal,sent over a network so as to secure or otherwise inhibit third-partyaccess to the content of the communication. In one embodiment, thepre-distortion may be performed (e.g., by a transmitter) in accordancewith a pre-distortion pattern or algorithm which is shared with onlyintended receivers. It should further be appreciated that thepre-distortion pattern may be used to vary the pre-distortion on aperiodic basis, as frequently as on a symbol-by-symbol basis. Thepre-distortion function may include distorting the phase and/or theamplitude of the signal's modulation. Without knowledge of the type andamount of pre-distortion applied, an unintended receiver will havedifficulty interpreting or otherwise demodulating the subjectcommunication. Other embodiments and aspects are disclosed and claimedherein.

As used herein, the terms “a” or “an” shall mean one or more than one.The term “plurality” shall mean two or more than two. The term “another”is defined as a second or more. The terms “including” and/or “having”are open ended (e.g., comprising). The term “or” as used herein is to beinterpreted as inclusive or meaning any one or any combination.Therefore, “A, B or C” means “any of the following: A; B; C; A and B; Aand C; B and C; A, B and C”. An exception to this definition will occuronly when a combination of elements, functions, steps or acts are insome way inherently mutually exclusive.

Reference throughout this document to “one embodiment”, “certainembodiments”, “an embodiment” or similar term means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the presentinvention. Thus, the appearances of such phrases or in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner on one or moreembodiments without limitation.

It should be appreciated that the invention and processes describedherein may be implemented using hardware, software or any combinationthereof (e.g., embedded logic), and the invention should not be limitedto any particular system design or implementation.

Exemplary Embodiments

FIG. 1A depicts a typical constellation diagram 100 for a BPSK modulatedsignal. BPSK, like all digital modulation schemes, utilizes a finitenumber of distinct symbols to represent digital data. In particular,BPSK uses a finite number of carrier wave phases, each assigned a uniquepattern of binary bits. In addition, each pattern of bits forms thesymbol that is represented by a particular phase. A receiver-sidedecoder or demodulator will determine the phase of the received signaland map it back to the symbol it represents, thus recovering theoriginal data. This requires the receiver to be able to compare thephase of the received signal to a reference signal. In addition tophase-shift keying, other shift keying types include amplitude-shiftkeying (ASK), frequency-shift keying (FSK) and (PSK).

BPSK modulation utilizes two phases—represented in FIG. 1A byconstellation points 110 and 120, respectively. The BPSK constellationpoints 110 and 120 are positioned with uniform angular spacing to impartmaximum phase-separation between adjacent points. In addition, they arepositioned on a circle so as to be transmitted using the same energy. Inthis fashion, the amplitudes needed to represent the data within thecarrier wave will be the same regardless of the data composition.

FIG. 1A includes the real and imaginary axes—referred to as the in-phaseand quadrature axes respectively. In short, the amplitude of each pointalong the in-phase axis is used to modulate a cosine (or sine) carrierwave and the amplitude along the quadrature axis to modulate a sine (orcosine) wave. In short, a BPSK symbol comprised of constellation point110 will denote a binary bit value of “1,” while a BPSK symbol comprisedof constellation point 120 will denote a binary bit value of “0”.

In reality, data may be inadvertently distorted due to degrading channelconditions, interference and the like. Thus, while FIG. 1A show a “1”bit as being 180 degrees on the I-axis and a “0” as being 0 degrees onthe I-axis, channel distortion can shift constellation point positionsand, as such, receivers are expected to be able to differentiate betweena “1” and a “0” with less than 90 degrees distortion.

By way of illustration, FIG. 1B depicts a constellation diagram 130 inwhich both constellation points 110 and 120 may be distorted by ±89degrees. For example, constellation point 140 represents a ±89 degreesphase distortion relative to the “1” bit, while constellation point 150represents a −89 degrees phase distortion relative to the “1” bit.Similarly, constellation point 160 represents a −89 degrees phasedistortion relative to the “0” bit, while constellation point 170represents a ±89 degrees phase distortion relative to the “0” bit.However, if the phase distortion exceeded 90 degrees, it would bedifficult for the receiver to distinguish between a “1” bit and a “0”bit.

In order to compensate for signal distortion (particularly phasedistortion greater than ±89 degrees), a preamble or header frame may beused. That is, a data stream will be preceded by a preamble comprised ofa string of predetermined bits. This string of predetermined bits isknown to the receiver and, as such, can be compared to a known referencepattern in order to determine the amount and direction of phasedistortion correction to apply to the incoming signal. The preamble maybe sent only once, or periodically in order to account for changingchannel conditions.

With the above provided by way of background, one embodiment of theinvention relates to pre-distorting the modulation of a digital datasignal such that only an intended receiver is able to discriminate thedistortion correctly. To that end, FIG. 2A depicts a BPSK diagram for apreamble 200 in accordance with one embodiment of the invention. Inparticular, a typical receiver using BPSK modulation would expect the“1” bit to be in or around position 210 and the “0” bit to be in andaround position 220. Upon receiving the preamble 200, a typical receiverwould compare the preamble 200 to a reference preamble and determinethat the actual signal distortion has rotated the constellation points230 and 240 by 45 degrees, as shown in FIG. 2A. A complimentary phasecorrection would then be applied to all subsequently-received data.

However, if a pre-distortion of 90 degrees is applied to the datapackets which follow the aforementioned preamble, the correction appliedby all unsuspecting receivers would erroneously interpret each “1” bitas a “0” bit and each “0” bit as a “1” bit. For example, the diagram ofdata frame 250 in FIG. 2B shows the actual positions of constellationpoints 260 and 270, which, based on the preamble 200 of FIG. 2A, wouldbe incorrectly rotated to positions 220 and 210, respectively. For anyreceiver which was not aware of the 90 degree pre-distortion, the frame250 would be interpreted incorrectly. In this fashion, the communicationlink between a transmitter and an intended receiver would be secured.

While in certain embodiments, the preamble itself (e.g., preamble 200)may also be pre-distorted in accordance with a pre-distortion pattern.It should further be appreciated that an undistorted preamble may besent instead and used by all available receivers to correct for actualchannel distortion in the normal course. However, the signal, whichwould follow such a preamble, may then be pre-distorted by thetransmitter such that the data is unreadable by any unintended receiver,despite the fact such unintended receivers have properly corrected foractual signal distortion in accordance with the undistorted preamblereceived.

While in one embodiment the pre-distortion may be applied only once,such as at the beginning of the transmission, it should further beappreciated that the amount or direction of the pre-distortion may varyperiodically, even as frequently as on a symbol-by-symbol basis. Whenthe pre-distortion varies, the variable distortion applied by thetransmitter may be generated in accordance with a pattern or algorithmwhich is shared with only intended receivers prior to transmission.

While the disclosure to this point has used BPSK modulation as the basisfor an exemplary embodiment, it should further be appreciated that theprinciples of the invention are equally applicable to any othermodulation scheme, such as QPSK, QAM, OFDM, etc. Additionally, thepre-distortion may include amplitude pre-distortion, either separatelyor in combination with the aforementioned phase pre-distortion. In stillanother embodiment, the communication link may be secured usingpre-emphasis or by varying the modulation scheme on a periodic basis,including on a packet-by-packet or symbol-by-symbol basis.

For those embodiments which vary the pre-distortion rapidly (e.g.,symbol-by-symbol), typical receivers will not be able to adjust quicklyenough to such radically varying channel conditions in order to properlydemodulate the received signal. In short, without advance knowledge ofthe distortion pattern unintended receivers will not be able to keep upwith the rapidly changing distortion. Thus, by providing an intendedreceiver with the pre-distortion pattern (e.g., direction and amount ofrotation), the communication link may be effectively secured.

Referring now to FIGS. 3A-3B, depicted QPSK constellation diagrams 300and 310 representative of another embodiment of the invention. Inparticular, FIG. 3A depicts a QPSK diagram 300 reflecting the expectedposition for each of the possible constellation points (i.e., 00, 10, 11and 01). However, FIG. 3B depicts a QPSK diagram 310 in which the signalhas been pre-distorted by rotating the possible constellation points by180 degrees. In this fashion, any unsuspecting receivers wouldincorrectly interpret the received symbols, and hence be unable to readthe data stream.

With reference now to FIG. 4A, depicted is one embodiment of a system400 for carrying out one or more aspects of the invention. Inparticular, system 400 includes a transmitter 405 in communication witha receiver 410 over a network 415, which may comprise a wireline orwireless network utilizing any known communication protocol, such as a802.11x or the like.

As shown, transmitter 405 includes at least an encoder 420 and amodulation circuit 425. In one embodiment, the encoder 420 encodes data,such as video data from a source (not shown) in order to provide digitalsignal 430 to some destination point (i.e., receiver 410) via a network415. In one embodiment, the encoder 420 may encode video content inaccordance with the H.264/AVC coding standard, or any other videoencoding standard. Once encoded, the digital signal 430 is provided tothe modulation circuit 425 for modulation and eventual transmission outover network 415. It should of course be appreciated that any videoencoding scheme may be used, and that the network 415 may comprise anywireline or wireless network, such as 802.11x wireless network.

In one embodiment, the modulation circuit 425 may also perform thepre-distortion operation as described above, and as will be described inmore detail below as well with reference to FIG. 5, wherein the phaseand/or amplitude of the outgoing signal is intentionally altered. In oneembodiment, the pre-distortion corresponds to a predetermined pattern oralgorithm.

Continuing to refer to FIG. 4A, digital signal 430 is transmitted vianetwork 415 to receiver 410. As shown, receiver includes at least ademodulation circuit 435 and decoder 440. Once the digital signal 430 isreceived by the receiver 410, the demodulation circuit 435 maydemodulate the received signal by undistorting the modulation of thedigital signal 430 as described above, and as will be described in moredetail below as well with reference to FIG. 6.

Once the signal is undistorted, the digital signal 430 may be decoded bydecoder 440 using the appropriate coding standard used by the encoder420 (e.g., H.264/AVC).

Although not depicted, it should equally be appreciated that thetransmitter 405 and/or receiver 410 may include other components, suchas a central processing unit (CPU) or other known controller circuitry.Moreover, as the invention may be implemented using any combination ofsoftware and hardware, the transmitter 405 and receiver 410 may includeembedded logic for carrying out the processes of the invention, asdetailed herein.

Other components of the transmitter 405 and/or receiver 410 may includerandom access memory, non-volatile memory (e.g., hard disk, floppy disk,CD-ROM, DVD-ROM, tape, high density floppy, high capacity removablemedia, low capacity removable media, solid state memory device, etc.,and combinations thereof). Additionally, the transmitter 405 and/orreceiver 410 may include a network interface (e.g., a network interfacecard, a modem interface, integrated services digital network, etc.) forcommunication over network 415.

It should be appreciated that system 400 may have numerous alternateconfigurations other than as depicted in FIG. 4A. For example, theencoder 420 may be separate from the transmitter 405 and/or the decoder440 need not be integrated into the receiver 410.

FIG. 4B depicts another embodiment of a system 450 configured toimplement one or more embodiments of the invention. In particular,system 450 includes a first transceiver 455 in communication with asecond transceiver 460 over the network 415. Since it is likely thatcommunication will be two-way, the first and second transceivers mayexchange the encoded data 430 over 415 in a back-and-forth manner. Inorder to provide two-way functionality, the first and secondtransceivers 455 and 460 may each include an encoder/decoder 465 and475, respectively, which are each configured to perform both encodingand decoding operations.

The first and second transceivers 455 and 460 are further depicted aseach including a modulation/demodulation circuit 470 and 480,respectively, for both modulating and demodulating the encoded data 430for transmission over the network 415. In this fashion, the firsttransceiver 455 may be configured to perform the pre-distortionoperation described above when transmitting data, as well as to performthe reverse distortion operation when receiving data. Similarly, thesecond transceiver 460 may similarly be configured to perform bothpre-distortion and corresponding reverse distortion operations whentransmitting and receiving data respectively.

Referring now to FIG. 5, depicted is one embodiment of a process to becarried out by the transmission side of a communication system (e.g.,system 400 or 450) configured in accordance with the principles of theinvention. In particular, process 500 begins at block 510 with areceiver being provided with a pre-distortion pattern or algorithm. Itshould be appreciated that the pattern may be provided to the receiverin any manner, so long as it precedes the reception of the pre-distortedsignal at issue. In one embodiment, the pre-distortion pattern mayinclude data representative of how a given modulation scheme is to bevaried prior to transmission such as, but not limited to, a distortionof the phase and/or amplitude of the signal modulation. Similarly, thepre-distortion pattern may include data representative of how amodulation scheme may be changed from one scheme to another (e.g.,change from BPSK to QPSK, from QPSK to QAM, from QAM to OFDM, etc.).

Process 500 may then continue to block 520 where the signal in questionis pre-distorted by a transmitter (e.g., during signal modulation), inaccordance with the pre-distortion pattern that was provided to thereceiver above in block 510. While in one embodiment, the pre-distortionincludes distorting the phase and/or the amplitude of the modulatedsignal, it should equally be appreciated that the signal may bedistorted in numerous other fashions, such as changing the modulationscheme.

While the operation of block 520 is depicted as being a singledistortion operation, it should be appreciated that such pre-distortionmay vary periodically during signal transmission, even as frequently ason a symbol-by-symbol basis. When the pre-distortion varies, thevariable distortion applied by the transmitter may similarly begenerated in accordance with the pre-distortion pattern of block 510.

Once distorted in accordance with the previously-describedpre-distortion pattern, process 500 continues to block 530 where thepre-distorted signal may then be transmitted out over a networkconnection.

FIG. 6 describes one embodiment of a process to be carried out by thereceiver side of a communication system (e.g., system 400 or 450)configured in accordance with the principles of the invention. Inparticular, process 600 begins at block 610 with the receiving of apre-distortion pattern or algorithm. It should be appreciated that thepattern may be provided to the receiver in any manner, so long as itprecedes the reception of a signal that has been pre-distorted inaccordance with the principles of the invention. As described above, itshould again be appreciated that the pre-distortion pattern of block 610may include data representative of how a given modulation scheme is tobe varied prior to transmission, such as, but not limited to, adistortion of the phase and/or amplitude of the signal modulation. Thepre-distortion pattern may alternatively include data representative ofhow a modulation scheme may be changed from one scheme to another.

Process 600 may then continue to block 620 where the signal preamble orheader frame(s) are received over the network. In certain embodiments,the digital data stream at issue will be preceded by a preamblecomprised of a string of predetermined bits.

Once a preamble has been received, a determination may then be made atblock 630 as to whether the modulation scheme of the preamble has beenintentionally pre-distorted or not. While in one embodiment, thisdetermination may be made by comparing the preamble to a reference, orby parsing the preamble for a pre-distortion flag or other indicator, itshould be appreciated that numerous other approaches may be similarlyemployed.

If it is determined at block 630 that the preamble modulation has beendistorted, process 600 may continue to block 640 where the preamble'smodulation may be undistorted in accordance with the informationcontained in the pre-distortion pattern received above at block 610. Inone embodiment, the undistorting operating of block 640 may be the sameas the operation to be described below with reference to block 670.

In any event, once an undistorted preamble is available, whether byvirtue of the preamble not having been distorted or by virtue of theundistortion operation of block 640, process 600 may continue to block650 where any channel-condition-based distortion may be corrected orotherwise compensated for. In one embodiment, the string ofpredetermined bits within the preamble may be compared to a knownreference pattern in order to determine the amount and direction ofphase and/or amplitude distortion correction to apply to the incomingsignal.

Once any actual channel-condition-based distortion has been corrected,process 600 may continue to block 660 where the pre-distorted digitaldata signal in question may be received. As previously mentioned, thepre-distorted signal may be received over any network, either wired orwireless, from a transmitter which has pre-distorted the modulation ofthe digital signal in accordance with a predetermined distortion (e.g.,the pre-distortion pattern).

Continuing to refer to FIG. 6, process 600 may then continue to block670 where the received signal may be undistorted in accordance with thepre-distortion pattern of block 610. In one embodiment, thisundistortion operation may comprise altering one or both of the phasesand amplitude of the signal's modulation in a inverse manner as thatapplied on the transmitter side. As previously mentioned, the data orinformation required to properly perform the undistortion operation ofblock 670 may be provided in the pre-distortion pattern received aboveat block 610.

While in one embodiment, the undistortion operation of block 670includes reversing the distortion in the phase and/or the amplitude ofthe modulated signal, it should equally be appreciated that the signalmay be distorted in numerous other fashions, such as changing themodulation scheme.

While process describes a single distortion operation, it should beappreciated that the such pre-distortion may vary periodically duringsignal transmission, even as frequently as on a symbol-by-symbol basis.When the pre-distortion varies, the undistortion operation performed atblock 670 may proceed in accordance with the pre-distortion pattern ofblock 610. Similarly, while process 600 includes the reception of only asingle preamble, it should equally be appreciated that preambles may besent periodically in order to account for changing channel conditionsand/or changing pre-distortion patterns.

While the invention has been described in connection with variousembodiments, it will be understood that the invention is capable offurther modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as, within the known and customary practice withinthe art to which the invention pertains.

What is claimed is:
 1. A method for providing secure digitalcommunications comprising: receiving a pre-distortion pattern configuredto secure a signal transmission prior to receiving a pre-distorted datasignal; receiving the pre-distorted data signal over a networkconnection corresponding to the signal transmission; undistorting atleast one of a phase and an amplitude of the pre-distorted data signalin accordance with two or more modulation schemes consecutively based onthe pre-distortion pattern, thereby securely receiving the signaltransmission; wherein the pre-distortion pattern comprises datarepresentative of how to change from one modulation scheme to anothermodulation scheme while undistorting the pre-distorted data signal. 2.The method of claim 1, wherein the pre-distorted data signal comprises avideo signal, and wherein the method further comprises decoding thepre-distorted data signal following the undistorting in accordance witha video coding standard.
 3. The method of claim 1, further comprisingdemodulating the pre-distorted data signal in accordance with one of abinary phase-shift keying scheme, a quadrature phase-shift keyingscheme, and a quadrature amplitude modulation scheme.
 4. The method ofclaim 1, further comprising receiving a preamble over the network, priorto receiving the pre-distorted data signal.
 5. The method of claim 4,wherein the preamble is distorted in accordance with the pre-distortionpattern.
 6. The method of claim 4, further comprising correcting atleast one of a phase and an amplitude of the received pre-distorted datasignal for channel-condition-based distortion using the preamble.
 7. Amethod for providing secure digital communications comprising: providinga pre-distortion pattern to an intended receiver prior to transmitting apre-distorted data signal, the pre-distortion pattern configured tosecure a signal transmission; distorting at least one of a phase and anamplitude of a data signal corresponding to the signal transmission inaccordance with two or more modulation schemes consecutively based onthe pre-distortion pattern; and transmitting, following the distorting,the data signal over a network to the intended receiver, therebysecurely transmitting the signal transmission; wherein thepre-distortion pattern comprises data representative of how to changefrom one modulation scheme to another modulation scheme whileundistorting the pre-distorted data signal.
 8. The method of claim 7,wherein the data signal comprises a video signal, and wherein the methodfurther comprises encoding the data signal prior to the transmitting inaccordance with a video coding standard.
 9. The method of claim 7,further comprising modulating the data signal in accordance with one ofa binary phase-shift keying scheme, a quadrature phase-shift keyingscheme, and a quadrature amplitude modulation scheme.
 10. The method ofclaim 7, further comprising transmitting an undistorted preamble, priorto the data signal, over the network to the intended receiver.
 11. Themethod of claim 7, further comprising transmitting a preamble, prior tothe data signal, over the network to the intended receiver, wherein thepreamble is distorted in accordance with the pre-distortion pattern. 12.A receiver comprising: a network interface configured to couple thereceiver to a network; and a demodulation circuit configured to, receivea pre-distortion pattern configured to secure a signal transmissionprior to receiving a pre-distorted data signal, receive a pre-distorteddata signal over the network corresponding to the signal transmission,and undistort at least one of a phase and an amplitude of thepre-distorted data signal in accordance with two or more modulationschemes consecutively based on the pre-distortion pattern, therebysecurely receiving the signal transmission; wherein the pre-distortionpattern comprises data representative of how to change from onemodulation scheme to another modulation scheme while undistorting thepre-distorted data signal.
 13. The receiver of claim 12, wherein thepre-distorted data signal comprises a video signal, and wherein themethod further comprises decoding the pre-distorted data signalfollowing the undistorting in accordance with a video coding standard.14. The receiver of claim 12, further comprising demodulating thepre-distorted data signal in accordance with one of a binary phase-shiftkeying scheme, a quadrature phase-shift keying scheme, and a quadratureamplitude modulation scheme.
 15. The receiver of claim 12, furthercomprising receiving a preamble over the network, prior to receiving thepre-distorted data signal.
 16. The receiver of claim 15, wherein thepreamble is distorted in accordance with the pre-distortion pattern. 17.The receiver of claim 15, further comprising correcting at least one ofa phase and an amplitude of the received pre-distorted data signal forchannel-condition-based distortion using the preamble.